During a health camp the daily time (in minutes) spent on physical exercise by a group of adults was recorded and grouped as follows.
a. Time (min): 0–10 | 10–20 | 20–30 | 30–40
Frequency: 5 | f | 15 | 10
b. It is known that the mean time of exercise for the group is 20 minutes.
(i) Determine the missing frequency f.
c. (ii) Using the completed table, calculate the population variance and hence the population standard deviation of the time spent on exercise.